In the twentieth century materials science has made great strides, particularly in the area of strong, lightweight members. The development of aircraft in the twentieth century has particularly spawned the development of lightweight structures which can withstand a great deal of stress and torsion without adding an undue amount of weight to an aircraft.
One of the most commonly used structures in the construction of aircraft are composite support members, particularly hat section stiffeners which commonly take the form of trapezoidal support members attached to the surfaces to which they provide support.
It is common in the state of the art to construct hat section stiffeners and composite supports from epoxy plastic interspersed with fibers of boron, graphite, or glass. In the formation of these structures the combination is usually heated to a temperature between 200 and 400.degree. F. in order to set and cure the composite. More recent developments include the use of thermoplastic composites which are formed into hat section stiffeners by heating a sheet of material which is placed over a mandrel defining the shape of the stiffener and the cooled.
Other composites are also set and cooled at temperatures more closely resembling room temperature.
In the making of composite stiffener sections, particularly those of the epoxy and fiberglass type, great difficulty has been encountered in providing mandrels about which the stiffener sections may be formed and which may be readily withdrawn from the completed stiffener section once the setting of the composite has been effected. It has proven difficult to provide suitable mandrel materials which satisfy the following criteria. Suitable mandrels must be able to withstand the temperatures encountered in curing the composite sections without either losing their rigidity or expanding an unacceptable amount due to the elevated temperature. Upon cooling the mandrel material should be easy to remove from the completed stiffening section. The mandrel must also be able to withstand the stress encountered in withdrawing the mandrel material from elongated stiffener sections which have been completed.
The difficulties encountered in providing suitable forms or mandrels for formation of composite stiffening sections has led the use of lightweight foam forms which remain within the stiffener section once it has been set. The use of such a scheme for forming composite stiffeners adds additional unnecessary weight to the completed structure since the foam makes no significant contribution to the structural integrity of the completed stiffener section but does add weight.
Solid forms of aluminum or plastic can be used to form short lengths of composite stiffeners but are difficult or impossible to remove when long lengths are required, particularly when the stiffener runs over an arcuate surface such as a wing or along the length of a fuselage.
Salt mandrels have been proposed but they have undesirable characteristics under the elevated temperatures commonly encountered in composite formation and require a laborious and time consuming step of dissolution and flushing upon completion of the composite.